1. Field of the Invention
The present invention relates to a metal-enclosed switchgear using vacuum-valve breakers.
2. Description of the Background Art
A conventional metal-enclosed switchgear using vacuum-valve breakers is provided with fittings, such as insulators, for supporting bus-side conductor strips on an inner wall of a sealed tank filled with an insulating gas as disclosed in Japanese Laid-open Patent Publication No. 1999-185577, for example.
One problem of conventional metal-enclosed switchgear systems is that it is difficult to reduce their physical size because there are provided insulators and other fittings for supporting the bus side conductor strips on the inner wall of the tank as stated above.
The insulators used in a metal-enclosed switchgear should meet at least the following two requirements:                (1) The insulator should withstand an electromagnetic impact force exerted by the bus-side conductor strips when a short-circuit current flows; and        (2) Although one end of the insulator supports the bus-side conductor strip to which a high voltage is applied and the other end of the insulator is fixed to the inner wall of the tank which is grounded, no insulation breakdown should occur between both ends of the insulator.        
As it is necessary to ensure compliance with the first requirement, or to maintain the insulator's impact force withstand capability, it is impossible to excessively reduce the diameter of the insulator. This makes it difficult to reduce the width of the conventional metal-enclosed switchgear, or its depth as illustrated in FIG. 3 appended to the aforementioned Patent Publication No. 1999-185577. More specifically, three insulators are arranged side by side in a horizontal plane in the metal-enclosed switchgear of the Publication, so that its width inevitably increases if insulators having a large diameter is used. Taking into account the fact that a specific amount of installation space should be allowed around each insulator, it is even more difficult to reduce the width of the metal-enclosed switchgear.
To ensure compliance with the second requirement, that is the need to prevent insulation breakdown, there is no alternative but to increase the longitudinal length of the insulator. The surface discharge-voltage of an insulator of a specific length is normally lower than the breakdown voltage of a column of gas of the same length. Therefore, when using an insulator, it is inevitable that its length is larger than the length of a column of gas having a corresponding breakdown voltage. Consequently, the size of the switchgear, particularly its depth (left-to-right dimension as illustrated in FIG. 3 appended to the aforementioned Patent Publication), tends to increase.